1. Field of the Invention
This invention relates generally to the preparation of nanosized-sized metallic particles having a unique surface morphology which provide advantages for Surface Enhanced Raman Spectroscopy (hereinafter referred to as “SERS”). The present invention, therefore, also indirectly relates to the field of Raman spectroscopy, and more particularly, to a sensor for detecting and identifying chemicals and biological components using SERS.
More particularly, the present invention relates to a method for forming finely divided particles of gold with a discrete size and a unique shape. The method comprises: 1) forming a dilute solution comprising a soluble metal salt, water, and surfactants dispersed within a bulk oil phase to form a stable water-in-oil microemulsion, 2) forming a second solution comprising a reducing agent (hereinafter referred to as the “reductant”), water, oil, and the surfactants provided in the first solution; and 3) mixing the first and second solutions together thereby initiating a reduction reaction between the metal salt and the reductant and to form a nanoscale powder of the metal species.
The metal nanoparticles thus formed are useful as a signal-enhancing substrate for analyte detection using Raman spectroscopy, an emission technique that involves inelastic scattering of incident laser energy and results in spectral peaks that are frequency shifted from the incident energy. The Raman bands arise from changes in polarizability in a molecule during spectroscopic interrogation at specific light wavelengths that produce a vibrational mode of molecular translational energy. As a result, virtually all organic molecules display a characteristic Raman emission that corresponds to their inherent molecular structure. Therefore, a Raman sensor would not be limited to a specific class of molecules as is the case for the laser induced fluorescence (LIF) sensor. The inherently high resolution of Raman spectra often permits the analysis of several components in a mixture simultaneously.
Moreover, the Raman effect can be significantly enhanced by bringing the Raman-active molecule(s) close (<5 nanometers) to a nanometer-scale roughened metal surface. Bringing molecules in close proximity to metal surfaces is typically achieved through adsorption of the Raman-active molecule onto suitably roughened gold, silver, copper or some other metallic substrate. Surface-enhancement of the Raman activity is also observed with metal colloidal particles, metal films on dielectric substrates, and metal particle arrays. The mechanism by which this surface-enhanced Raman scattering (referred to hereinafter as “SERS”) occurs is not well understood. It is thought to result from a combination of (i) electromagnetic effects known as surface plasmon resonances structures that occur at the metal-analyte interface and enhance the local intensity of the light, and (ii) chemical effects known as the formation and subsequent transitions of charge-transfer complexes between the metal surface and the Raman-active molecule.
SERS thereby allows for the enhanced detection and identification of molecules that are attached or brought into close proximity to the surface of a metal nanoparticle. A Raman-enhancing metal that is associated or bound to a Raman-active molecule(s) is referred to as a SERS-active nanoparticle. Such SERS-active nanoparticles can have utility as optical tags. For example, SERS-active nanoparticles can be used in immunoassays when conjugated to an antibody against a target molecule of interest. If the target molecule is immobilized on a solid support, then the interaction between a single target molecule and a SERS-active nanoparticle bound to an antibody can be detected by searching for the Raman spectrum unique to the Raman-active molecule. Furthermore, because a single Raman spectrum (having wavelengths from 100 cm−1 to 3500 cm−1) can detect many different Raman-active molecules, different SERS-active nanoparticles can be used in multiplexed assay formats.
The powders described herein also find utility as catalysts such as those used in electrochemical applications, including fuel cells and the like, automotive applications such as automotive catalytic converters, and other similar applications. The instant invention is particularly drawn to a method for forming small particles subsequently dispersed onto an inert support means such as finely divided ceramic particles or molecular sieves. Commonly known methods for loading catalyst particles onto such a catalyst support means include precipitation, adsorption, ion exchange, electrostatic adsorption, and physical impregnation through spray and/or dip coating.
2. Related Art
U.S. Pat. No. 6,770,488 discloses a colloidal system for detection of a variety of analytes involving techniques which permit reconstitution of a desiccated substance such as for surface enhanced Raman spectroscopic analysis and multiple sensors at once, each having different spectra through the use of markers or the like.
U.S. Pat. No. 6,348,431 discloses a process for producing fine, contamination-free, noble metal alloys ranging in size from about 5 nm to about 500 nm. The process comprises forming a solution of organometallic compounds by dissolving the compounds into a quantity of a compatible solvent, mixing a portion of each solution to provide a desired molarity ratio of ions in the mixed solution, rapidly quenching droplets of the mixed solution to initiate a solute-solvent phase separation as the solvent freezes, removing said liquid cryogen, collecting and freeze-drying the frozen droplets to produce a dry powder, and finally reducing the powder to a metal by flowing dry hydrogen over the powder while warming the powder to a temperature of about 150° C.
U.S. Pat. No. 5,686,150 discloses a process for depositing metals onto various substrates. In particular the patent teaches various salts of platinum, palladium and ruthenium deposited on substrates such as organic polymers, hybrid polymers and carbons. Said salts are then reduced by exposing the substrate with an energy source and then contacting the exposed substrate to a developer to permit reduction of the metal.
U.S. Pat. No. 5,514,202 discloses a method for producing fine alloy powders of silver and palladium at temperatures at or below 50° C. The method comprises forming solutions of the metal nitrates, mixing them, and then adding a neutralizing and complexing agent to the mixed solution to adjust the solution pH to between 2.5 and 3.5. This resultant solution comprises silver and palladium ions. A solution comprising a reducing agent is then prepared and brought into contact with the mixed solution while stirring the solutions and maintaining the solution temperature between 15° C. and 50° C. in order to allow the silver and palladium ions to be reduced and to co-precipitate and form silver-palladium alloy particles.
U.S. Pat. No. 4,721,524 teaches a method for producing a non-pyrophoric submicron alloy powder of Group VIII metals. An aqueous chemical process involving spontaneous nucleation is employed in the manufacture of the alloy product. One such product is prepared with isomorphic compounds of nickel and palladium. In the preferred form, small concentrations of palladium and/or platinum ions are added to ionic nickel and/or cobalt solutions. A hot solution of the metal ions of nickel and/or cobalt and palladium and/or platinum is mixed with hydrazine in a hot basic aqueous solution and rapidly diluted with hot water. The precipitate formed is filtered, sequentially chemically washed, and dried. The dried alloy product is a black, non-pyrophoric, magnetic powder. Examination by electron probe and X-ray diffraction shows the alloy product to be a solid solution of the constituent metals.
U.S. Pat. No. 4,145,214 discloses a photo-conductor composition prepared by a co-crystallization technique. In particular, this invention teaches that the photo-conductor can be formed by co-crystallization of the two components (p-terphenyl and p-quaterphenyl) from solutions in a common solvent. Typically the co-crystalline material is precipitated, for example, by subsequent evaporation of the solvent.
U.S. Pat. No. 3,357,819 discloses a process for preparing homogenous powders composed of ultra-fine particles. A solution or dispersion of a salt is freeze dried by dripping into a cold solution such as liquid nitrogen, followed by sublimation of the water from the particles. The patent does not refer to any pre-process method for controlling the shapes of the particles so generated.
Many other examples exist of alloy formation by similar solution-precipitation processes but, as will be shown, none exhibit the essential characteristics of the instant invention.